This invention relates to a method and device for controlling an apparatus and more particularly to a method and device for controlling an apparatus, hereinafter referred to as "a three state apparatus", which has three states of operation, for example, off, high load, and low load.
An example of such an apparatus is an air compressor and the invention will be described hereinafter in relation to an air compressor, although it will be appreciated that the invention may be used to control any other desired apparatus having three operational states.
An air compressor with which the invention may be used, has a first actuating element comprising an electrically powered motor, a reservoir for compressed air, and a second actuating element comprising a valve means. When the motor is switched off, the compressed air in the reservoir may still be used, but the reservoir is not replenished. When the motor is switched on, either the valve means is operated in a high load position so that the operation of the motor replenishes the reservoir, so that the motor and hence air compressor is operating in a high load state, or the valve means is operated in a low load position so that although the motor continues to operate, the reservoir is not replenished and the motor and hence the air compressor therefore operates in a low load state.
Control devices are known to switch operation of air compressors from one state to another, in response to a signal received from a pressure sensor which senses the air pressure in the reservoir.
These control devices usually operate in one of two ways.
First, when the motor is operating in a high load state and the pressure sensed reaches a first, higher, predetermined pressure, the device switches the motor to the off state until a second, lower, predetermined pressure is sensed, when the motor is returned to its high load operational condition.
Second, when the motor is operating in a high load state, and the pressure sensed reaches a first, higher, predetermined pressure, the device signals the valve means to switch from the high load position to the low load position so that the motor continues to operate but in a low load state.
Obviously, the first manner of operation is generally the more economical at least where the motor is switched off for reasonably long periods because the motor does not continue to consume power whilst the air pressure in the reservoir falls from the first to the second predetermined pressure.
However, where for example air from the reservoir is being used at a rate only slightly less than the rate of replenishment of the reservoir, the motor is frequently switched on and off. This is undesirable as this can lead to damage to the windings of the motor through which large transient electrical currents in excess of normal operating currents, would frequently flow, causing the windings to overheat. Further, this is uneconomical.
It is therefore an object of the present invention to provide a new or improved method for controlling a three state apparatus, which overcomes or reduces this problem.